Jeff Howell advises readers on a joist condundrum and whether or not
crossover plumbing is a good idea.

Q Some of our first-floor joists have been notched in the past to take pipes, mainly not too far from supporting walls, and I would like to strengthen them if possible. The joists do not appear to have sagged. I have researched on the internet and seen suggestions to attach metal plates either side of the joists and bolt through them, or to glue and screw pieces of 18mm plywood to the joists.

It seems to me that the bolting required in the first option might further weaken the joists. The plywood, glue and screw suggestion therefore appeals on the basis that the strengthening becomes more of an integral part of the joist. Where pipes are still in situ, the strengthening would not be to the full depth of the joist but would seem to be better than nothing. It seems to me that rigid material fixed on the top edge of the joist, over the notch, would provide better compressive strength at that point, but this would interfere with the floor covering.

TS, by email

A Notching the tops of timber-floor joists to fit plumbing pipes can weaken them, but you are fortunate in that your notches are close to the supporting walls, in line with best practice. A joist under load flexes most near its centre, so notches should be restricted to a zone no closer than one quarter of the span from the end. More importantly, notches should be no deeper than one eighth the depth of the joist, meaning that a 150mm (6in) joist — common in some older housing – should be notched 19mm at most. This restriction is frequently ignored by plumbers installing standard 22mm copper pipes for central heating.

As I have written before, some plumbers do seem to view joists as inconvenient obstacles to their pipe runs, and attack them with scant regard to their structural integrity. I have even seen floor joists in Victorian houses notched to take 40mm (1.5in) waste pipes from baths and washbasins — it’s a wonder there isn’t more structural damage due to this practice.

For redundant notches that no longer carry pipes, the simplest and most elegant strengthening solution is to cut a block of timber to make a tight fit, spread wood glue on the joining surfaces and tap it into place. For notches that still carry pipes, you can line them with purpose-made pipe-joist clips from Pegler Yorkshire fittings. These are made from rigid polypropylene and are slightly wedge-shaped, so you tap them down over the pipe and screw them in place to fill the notch and stop it from compressing.

They have a zinc plate on top to provide extra protection

against nails or screws accidentally piercing the pipe, and are available in 15mm and 22mm single and double sizes, and a “mixed double” size to take a 15mm and 22mm pipe side by side. This is a truly useful innovation, which I’m surprised isn’t more widely stocked by plumbers’ merchants. Call 0800 156 0010 for stockists.

Cross about crossover plumbing

Q When I bought my house, I did not check the lavatories during the viewing, because it was inhabited and the owner was following me around. Now I have moved in, I find that the plumbing for the upstairs loo is attached to the hot-water system. My very reliable plumber has spent a long time trying to trace the fault, and says it would be far too expensive to fix. Is this crossover plumbing going to be a good breeding place for bugs etc, or is it safe to be left as it is?

FC, Walsall

AI doubt that the hot-water feed to your WC cistern will present a health risk. In fact, your hot-water cylinder should be programmed to reach 60C at least once a day in order to kill bacteria. But clearly using hot water to flush the lavatory is going to waste a lot of money on your fuel bill (even though in the past, some readers have recommended it as a way of preventing condensation on the WC cistern!)

I can’t imagine why it would be all that costly to reconnect the pipework to the cold-water feed, unless perhaps the pipes are buried below expensive bathroom tiling. If that is the case, I suggest you ask your plumber if he couldn’t run-in a new separate cold-water pipe to the cistern. This new pipe could be surface-mounted to avoid having to disturb the tiling.